For liquid crystal display devices or other similar devices, the AC driving is conventionally adopted to suppress deteriorations of liquid crystal elements (electro-optical elements). In the AC driving, however, when reversing polarities of voltages for gradation display, it is required for a data signal line driving circuit to charge them to a voltage for gradation display as desired after discharging a data signal line and a pixel capacitor by inputting charges in reverse polarity, which results in large amount of power consumption. As a typical conventional technique to counteract this problem, for example, Japanese Unexamined Patent Publication No. 9-212137 (Tokukaihei 9-212137, published on Aug. 15, 1997) discloses the following driving method.
FIG. 12 is a block diagram which schematically illustrates the structure of Japanese Unexamined Patent Publication No. 9-212137. In this conventional technique, the frame inverse driving method is adopted wherein voltages for gradation display in mutually reverse polarities are output between adjacent frames. Further, to suppress an occurrence of flicker, the line inverse driving method wherein voltages for gradation display in mutually reverse polarities are applied between pixels adjacent in the data signal line direction, and the dot inverse driving method wherein voltages for gradation display in mutually reverse polarities are applied between adjacent pixels in the scanning signal line direction are also adopted in combination with the above-mentioned frame inverse driving method.
In the foregoing driving method, the polarity of the display data is switched at every frame, for example, between the frame of FIG. 13(a) and the frame of FIG. 13(b). FIG. 13(a) and FIG. 13(b) respectively show portions corresponding to 8×6 pixels of a liquid crystal panel. When comparing FIG. 13(a) and FIG. 13(b), it can be seen that the polarities of all the pixels are switched at every frame, and in each frame, polarities of adjacent pixels in the data signal line direction (up-and-down direction in FIG. 13(a) and FIG. 13(b)) are switched, which indicates that the line inverse driving method is performed. Further, polarities of adjacent pixels in the scanning signal line direction (left-to-right direction in FIG. 13(a) and FIG. 13(b)) are switched, which indicates that the dot inverse driving method is performed.
As illustrated in FIG. 12, separation switches s1, s2, . . . , sn are provided in data signal lines d1, d2, . . . , dn connected to a data driver 1 respectively, and further, between adjacent data signal lines d1, d2, . . . , n, short switches sw1, sw2, . . . , swn-1 are provided for short circuiting the data signal lines at downstream sides of the separation switches s1 to sn. When scanning respective scanning signal lines in order, and applying voltages for gradation display for data signal lines d1 to dn to pixel capacitors via respective switching elements of the pixels, the separation switches s1 to sn conduct, and the short switches sw1 to swn-1 are cut off.
On the other hand, directly before a voltage for gradation display is applied to each pixel, a blanking period is set where separation switches s1 to sn are cut off, and short-circuit switches sw1 to swn-1 conduct. As a result, the pixel capacitors in pixels on a line to be subjected to the selection scanning are short-circuited by short-circuit switches sw1 to swn-1 via data signal lines d1 to dn from respective switching elements of the pixels, and positive charges and negative charges which exist substantially evenly are neutralized to be the same potential. Here, the cut-off of the switches s1 to sn does not adversely affect the short-circuiting of the output stage of the data driver 1.
According to the foregoing driving method, the data driver 1 is only required to charge the respective pixel capacitors to voltages for gradation display reversed from the neutralized, and it is therefore possible to suppress the power consumption of the data driver 1.
In general, in the liquid crystal panel, the number of data signal lines is twice as large as the number of the scanning signal lines. For example, for a compact-size liquid crystal panel for portable phones, the number of data signal lines is 168, while the number of scanning signal lines is 80. This is because, lines corresponding to R, G, B display data outputs for color display are provided for the data signal lines. In the foregoing conventional techniques, it is required to provide a large number of output terminals in the data driver 1, and also to provide short-circuit switches sw1 to s2n in the data driver 1 besides the separation switches s1 to sn, which results in another problem of increasing an area of an IC chip for the data driver 1. Moreover, the further is positioned the data signal line in selection-scanning order from the top, i.e., from the data driver 1, the longer is the distance between the data signal line d1 to dn and short-circuit switch sw1 to swn-1. Therefore, it is more likely that charges cannot be neutralized completely due to a drop in voltage caused by a large wiring resistance, and the power consumption cannot be reduced significantly. Further, as the lines becomes longer, a response time becomes longer due to dull waveform. Therefore, the effect of suppressing the power consumption would be small for a large screen display device in which long data signal lines are provided.